In recent years, a hierarchical virtual storage system has been known in which a high-capacity and inexpensive recording medium, such as a magnetic tape, is used as a back-end library apparatus, and also a recording medium with a higher access speed, such as an HDD (Hard Disk Drive), is used as a cache apparatus. A control apparatus that controls access to the cache apparatus and the library apparatus is provided in the virtual storage system, and it makes a host apparatus virtually recognize data stored in the cache apparatus as data of the library apparatus. As a result, the host apparatus utilizes a high-capacity memory area provided by the library apparatus as if it is connected to the host apparatus itself.
Meanwhile, as a technology for preventing loss of data stored in a storage system or the like, a technology that makes data redundant using parity has been known. As an example of such technology, there is a technology in which data to be backed up is divided into certain size of data, parity is generated from the plurality of pieces of divided data, and the plurality of pieces of divided data and parity are respectively recorded in different backup apparatuses (for example, see Japanese Laid-open Patent Publication No. 2003-150325). In addition, there is a technology in which a striping unit in RAID (Redundant Arrays of Inexpensive Disks)-5 is made variable according to a content of data to be record (for example, see Japanese Laid-open Patent Publication No. 2003-150323). Furthermore, there is a technology in which exclusive OR operations for generating parity are collectively executed by a disk apparatus for storing the parity (for example, see Japanese Laid-open Patent Publication No. 10-312246).
In the above-described virtual storage system, there is a request for reducing a time required to read data from a magnetic tape to a control apparatus. For example, when data is read from the magnetic tape, a transport mechanism transports to a tape drive the magnetic tape in which the data has been stored, then mounts it therein, and thus it takes a long time before read of the data itself is started. In addition, there is a case where a temporary read error occurs during read of the data from the magnetic tape, and in that case, it takes an extra time of retrying the read. Furthermore, there is also a problem that a read speed itself from the magnetic tape is low as compared with a read speed from an HDD or the like.